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SPACE SCIENCE
Salvaging A Very Expensive Phone Call From Titan

Surface map of Titan. From October 4 to 18, 1994, the Hubble Space Telescope Planetary Camera took 53 images of Titan at wavelengths ranging from the ultraviolet to the near-infrared. Fourteen of those images have been used to make the first albedo map of Titan's surface, at 0.94 microns; 8 others show Titan's surface near 1.08 microns. A paper (abstract) has been prepared and submitted for publication by Peter Smith, Mark Lemmon, Ralph Lorenz, John Caldwell, Larry Sromovsky, and Michael Allison.
by Bruce Moomaw
Cameron Park - March 20, 2001
In short, a spectacular mission -- but last October, the European Space Agency admitted a serious and embarrassing problem due to a a glaring design error which no one had caught during years of design reviews and ground tests.

It now turns out that the Huygens data receiver on Cassini isn't properly adjusted to the Doppler change in the frequency of Huygens' received signal caused by the fact that Cassini will be barreling toward Titan (and Huygens) at over 21,000 km per hour during its listening period.

The tracking signal -- whose Doppler frequency Cassini will monitor to track how fast the probe is being blown sideways by Titan's winds -- will come in fine; but the carrier signal for the probe's actual data telemetry may be 10 times fainter than expected!

There are a number of possible actions that can alleviate this mess. For one thing, Cassini could be re-aimed so that, during the flyby, its closest approach to Titan is 10,000 to 20,000 km away from the moon rather than a mere 1200 km -- meaning that during the listening period, it would be flying past the spot on Titan where Huygens is coming down on a distant parallel line rather than barreling almost straight toward it, thus reducing its straight-line approach speed to Huygens and the resultant Doppler effect.

However, everyone would like to avoid this if at all possible. That first flyby is also designed to use Titan's gravity to slow Cassini down so that its second orbit around Saturn is much less elongated and takes only 48 days -- and that flyby, in turn, is precisely timed to initiate Cassini's whole 4-year series of repeated Titan flybys that will veer it into constantly changing orbits around Saturn, which in turn have been very carefully chosen to give it the best possible overall scientific observations of Saturn, its rings, and its smaller moons.

Scientists took literally years to finally select this optimum orbital tour design ("Tour 18-5") -- and they are extremely reluctant to have to dump it and start choosing a new tour all over again.

Fortunately, a frantic ESA study concluded in December that there are other less drastic measures that can mostly -- and perhaps completely -- correct the problem. For one thing, Huygens' telemetry format may be reprogrammed to include a greater scattering of blank "zero packets".

These may slightly reduce the total amount of scientific data that it transmits twice for redundancy -- meaning that a small amount of its scientific telemetry may be less reliable in its accuracy than would otherwise be the case -- but it can also double the clarity with which Cassini's receiver can decode Huygens' signal.

The exact rate at which Huygens switches back and forth between its two radio antennas as it rotates beneath its parachute could also be adjusted.

The most important correction, though, will probably have to do with the distance by which Cassini trails Huygens as the two hurtle toward Titan. The original plan was to have Huygens enter Titan's atmosphere and start transmitting data while Cassini was still 4 hours away from Titan, so that at the end of the 3-hour listening period Cassini was still an hour away from its Titan flyby.

Since Cassini will listen for Huygens' signal with its big main high-gain antenna dish -- which is rigidly mounted on Cassini's top -- this would avoid the need for Cassini to slew around to keep tracking Huygens during the last part of Cassini's Titan flyby.

However, Cassini's deflection maneuver after releasing the Huygens probe will now probably be modified so that it trails the probe by as little as 3 hours, thus hurtling past Titan just as the listening period ends.

This means that, during the last hour or so of the listening period, Cassini will have indeed have to slew around to keep its antenna pointed at Huygens' descent point on Titan -- but while this moderately complicates Cassini's listening procedure, it also means that at the very start of Huygens' descent (when Cassini is most distant from it), the strength of Huygens' received signal will again be almost doubled.

It does also mean that the plan to have Cassini slew around again after the Huygens listening period is over -- to spend an hour pointing the cameras and spectrometers rigidly mounted on its side at Huygens' landing spot to observe local weather patterns while Cassini continues to barrel toward the moon -- may have to be dropped.

Finally, while Cassini's orbital tour itself -- and the exact distance and position of its 44 planned flybys of Titan -- will probably not be changed, the exact point during that tour at which it dumps Huygens into Titan's atmosphere very well may be.

From the start, Cassini's second close flyby of Titan (on Jan. 14, 2005) has been planned as a backup opportunity to drop off Huygens in case some problem prevented it on Cassini's first flyby of the moon.

If this path is chosen, Cassini can instead devote its first Titan flyby to observing Titan's weather patterns and determining just how fast Titan's upper-air winds blow, and even whether they blow west or east.

Its cameras and its Visual and IR Mapping Spectrometer ("VIMS") can peer through the orange haze to track any methane cloud patches below it, and its spectrometers can even make Doppler measurements of the speed with which the upper haze is being blown along.

This will allow Cassini to more accurately point its high-gain dish at the exact spot on Titan where Huygens is likely to be located at any moment, again greatly increasing the strength of the received signal, especially during the last part of the descent.

Cassini might even delay its Huygens release until its third Titan flyby on Feb. 15, allowing still more advance observations of Titan's winds and exact measurements of Titan's orbital position. All this could raise the strength of the received signal at the end of Huygens' descent as much as fivefold.

And on top of that, the frequency of Cassini's relay receiver is very slowly drifting -- accidentally, but in just the right direction (of it continues) to cancel out much of the Doppler reception problem!

At any rate, the European Space Agency seems justified in its optimism that it can solve the signal-reception problem, and that Huygens will be able to send back most or all of its data, revolutionizing our understanding of this utterly mysterious world.

But while Huygens' story will be over, Cassini's travels about Saturn will be just beginning. In the next part of this report, I'll review the details of that epic 4-year tour around Saturn, and the scientific observations that Cassini will make of Saturn, its rings, and countless moons.

Back To Part One

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SPACE SCIENCE
A Distant World of Ice and Hydrocarbons
Cameron Park - February 27, 2001
In the first of our series on the activities of the Cassini spacecraft at Saturn, we ended with the spacecraft having entered orbit and firing its main engine to fly by Saturn's moon Titan, where it will parachute its European-constructed probe Huygens into that moon's atmosphere, giving us our first close up look at Saturn's most enigmatic moon.



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